Methods for in situ deposition of coatings and articles produced using same

ABSTRACT

Methods for depositing a coating on a metal surface can include heating a metal surface to a temperature not greater than its melting point; while heating the metal surface, applying a vacuum thereto; and while heating the metal surface, releasing the vacuum and backfilling with a first purge gas, where the first purge gas is reactive with the heated metal surface so as to deposit at least one layer of a coating thereon. The present methods can be used to deposit a coating in situ during the fabrication of solar receivers, in which the solar receivers contain an annulus defined by a metal tube as the inner surface and a material that is at least partially transparent to solar radiation as the outer surface.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. §119 from U.S. Provisional Patent Application Ser. No. 61/385,899, filed Sep. 23, 2010, which is incorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

FIELD OF THE INVENTION

The present invention generally relates to coatings, and, more specifically, to methods for producing coatings.

BACKGROUND

Coatings are frequently used in a variety of applications to protect the materials beneath the coating from environmental exposure and/or to modify other physical properties of the materials beneath the coating. Physical properties that can be altered by a coating can include, but are not limited to, optical properties, thermal properties and mechanical properties. More specifically, thermal and electromagnetic absorption and emission properties of an article can be profoundly influenced by the presence of even a vanishingly thin layer of a coating deposited thereon.

A number of different techniques have been developed for depositing thin layer coatings. These techniques can include, for example, sputtering, evaporative deposition, pulsed laser desorption, electroplating, electroless plating, chemical vapor deposition, and the like. In the manufacture of articles, most of these coating techniques have to be conducted separately from other manufacturing steps used in the fabrication of the article. Further, many of these deposition techniques can require specialized equipment that can increase the time and expense required for fabricating an article containing a coating.

In view of the foregoing, simple, low cost techniques for producing a coating, particularly during the fabrication of an article, would be of substantial benefit in the art. The present disclosure satisfies this need and provides related advantages as well.

SUMMARY

In some embodiments, methods for depositing a coating on a metal surface are described herein. The methods include heating a metal surface to a temperature not greater than its melting point; while heating the metal surface, applying a vacuum thereto; and while heating the metal surface, releasing the vacuum and backfilling with a first purge gas. The first purge gas is reactive with the heated metal surface so as to deposit at least one layer of a coating thereon.

In some embodiments, methods for depositing a coating on a solar receiver are described herein. The methods include applying a vacuum to an annulus having an outer surface defined by a material that is at least partially transparent to solar radiation and an inner surface that is defined by a metal tube; heating the metal tube, while applying the vacuum thereto, to a temperature not greater than its melting point; and, while heating the metal tube, releasing the vacuum and backfilling with a first purge gas, where the first purge gas is reactive with the heated metal tube so as to deposit at least one layer of a coating thereon.

In some embodiments, solar receivers can be prepared by a process that includes applying a vacuum to an annulus having an outer surface defined by a material that is at least partially transparent to solar radiation and an inner surface that is defined by a metal tube; heating the metal tube, while applying the vacuum thereto, to a temperature not greater than its melting point; and, while heating the metal tube, releasing the vacuum and backfilling with a first purge gas, where the first purge gas is reactive with the heated metal tube so as to deposit at least one layer of a coating thereon.

The foregoing has outlined rather broadly the features of the present disclosure in order that the detailed description that follows can be better understood. Additional features and advantages of the disclosure will be described hereinafter, which form the subject of the claims.

BRIEF DESCRIPTION OF THE DRAWING

For a more complete understanding of the present disclosure, and the advantages thereof, reference is now made to the following description to be taken in conjunction with the accompanying drawing describing a specific embodiment of the disclosure, wherein:

FIG. 1 shows a schematic of an illustrative solar receiver.

DETAILED DESCRIPTION

The present disclosure is directed, in part, to methods for depositing coatings on a metal surface. The present disclosure is also directed, in part, to metal surfaces having a coating deposited thereon, particularly solar receivers.

Although coatings are frequently used with great utility in a wide variety of applications, techniques for depositing coatings can considerably add to the time and expense required to fabricate articles containing a coating. The methods described herein can advantageously address these shortcomings in the art by providing simple techniques for preparing coatings on a metal surface. More particularly, in certain cases, the coating methods described herein can be used for the in situ deposition of a coating on a metal surface. That is, during the fabrication of an article, a coating can advantageously be applied to the article using simple modifications of at least some of the operations that are already being used for the fabrication of the article. For example, coatings can be applied to articles according to the present methods when a vacuum bakeout (e.g., a hydrogen bakeout) is used during fabrication of the article.

One example of an article containing a metal surface in which a coating can be deposited in situ during the article's fabrication is a solar receiver. FIG. 1 shows a schematic of an illustrative solar receiver 100. Solar receiver 100 can be used as the thermal energy collector in a parabolic trough solar receiver array, in which solar receiver 100 can absorb thermal energy from focused solar radiation, while emitting as little heat as possible back to the ambient environment. Illustrative solar receiver 100 contains an inner metal tube 110, which has a heat transfer fluid (e.g., an oil or like high boiling fluid) flowing through its interior space in order to carry collected heat away from solar receiver 100. In order to maximize the amount of collected heat and reduce thermal emission, inner metal tube 110 is typically surrounded by a vacuum. To this end, solar receiver 100 contains an outer surface 120 that is at least partially transparent to solar radiation (e.g., a glass), which defines annulus 115 together with inner metal tube 110 and contains the vacuum. Vacuum can be applied to annulus 115 through port 130. Most often, inner metal tube 110 is modified with a coating that increases its thermal absorptivity.

During the fabrication of typical solar receivers, inner metal tube 110 is heated using a heater (not shown), while applying vacuum to annulus 115 in order to achieve outgassing and desorption of species that would otherwise degrade the applied vacuum and potentially alter the thermal absorptivity. However, it has been advantageously discovered according to the embodiments described herein that if, instead of sealing annulus 115 to maintain the vacuum therein, the vacuum is broken and annulus 115 is backfilled with a purge gas while heating inner metal tube 110, an in situ coating can be applied to inner metal tube 110. Thereafter, fabrication of solar receiver 100 can be completed simply by re-applying vacuum to annulus 115, followed by sealing to maintain the vacuum. Therefore, the present methods offer the opportunity to simply modify the surface of inner metal tube 110 with a coating, using only simple modifications of the operations already in place for fabrication of a solar receiver.

As used herein, the term “vacuum” will refer to any pressure that is less than atmospheric pressure. Unless otherwise specified, the term vacuum should not be construed to constitute any particular magnitude of vacuum. In some embodiments, a suitable vacuum can be about 1×10⁻⁵ ton or lower. In other embodiments, a suitable vacuum can be about 1×10⁻⁶ ton or lower.

As used herein, the term “coating” refers to a material on a metal surface that is at least a monolayer in thickness. Illustrative coatings that can be applied to a metal surface according to the methods described herein include, but are not limited to, metal oxide coatings, metal nitride coatings, metal carbide coatings, and metal fluoride coatings. Unless otherwise specified, the term coating should not be construed to constitute any particular type of coating or any particular number of layers in the coating.

In some embodiments described herein, methods for depositing a coating on a metal surface can include heating a metal surface to a temperature not greater than its melting point; while heating the metal surface, applying a vacuum thereto; and while heating the metal surface, releasing the vacuum and backfilling with a first purge gas that is reactive with the heated metal surface so as to deposit at least one layer of a coating thereon. In some embodiments, the methods can further include depositing at least one layer of a coating thereon.

In general, any type of metal surface can be modified with a coating according to the embodiments described herein. In this regard, both pure metals and metal alloys can be used. In various embodiments, suitable metals can include, without limitation, titanium, copper, iron, aluminum, tungsten, and any combination thereof. Other suitable metals can be envisioned by one having ordinary skill in the art. In some embodiments, the metal surface can be polished to remove native oxides therefrom, in order to promote deposition of the coating. In some embodiments, the metal surface can be a metal tube such as, for example, in a solar receiver. In particular, in the case of a metal tube used in solar receiver, steels such as, for example, stainless steel, carbon steel or a combination thereof can be used.

Although certain embodiments of the present disclosure have been described in the context of a solar receiver, it is to be understood that any type of article having a metal surface can be modified with a coating according to the embodiments described herein. That is, the description herein regarding a solar receiver should be considered to be illustrative in nature and not limiting. More particularly, it is to be understood that any type of article having a metal surface can be modified with a coating according to the embodiments described herein by heating the metal surface and either applying vacuum to the metal surface directly (e.g., via an annulus, cavity or like opening in the article) or indirectly (e.g., by placing the article or a portion thereof in a heating device such as a vacuum furnace, for example, that can be placed under vacuum and backfilled with a purge gas thereafter).

In various embodiments, methods for depositing a coating on a metal surface of a solar receiver can include applying a vacuum to an annulus having an outer surface defined by a material that is at least partially transparent to solar radiation and an inner surface that is defined by a metal tube; heating the metal tube, while applying the vacuum thereto, to a temperature not greater than its melting point; and while heating the metal tube, releasing the vacuum and backfilling with a first purge gas that is reactive with the heated metal tube so as to deposit at least one layer of a coating thereon.

In the case of a solar receiver, various materials can be used to form the outer surface of the annulus. In general, the material forming the outer surface of the annulus needs to be at least partially transparent to solar radiation so as to allow the solar radiation to impinge upon the inner surface defined by the metal tube. Further, the material forming the outer surface of the annulus typically needs to have at least some degree of resistance to deformation under heating, since considerable heat can be generated when solar energy is focused upon the solar receiver. In some embodiments, a suitable material that is at least partially transparent to solar radiation can be a glass. In some embodiments, the glass can further include an anti-reflective coating adapted to minimize reflection therefrom so as to maximize the amount of solar radiation transmitted to the metal tube.

After depositing the coating on the metal tube of the solar receiver according to the methods described herein, fabrication of the solar receiver can be simply completed by continuing with standard fabrication operations. To this end, in some embodiments, after depositing the coating and while heating the metal tube, the methods can further include re-applying a vacuum to the annulus. In some embodiments, the methods can further include sealing the annulus so as to maintain the vacuum therein and complete the fabrication of the solar receiver. In alternative embodiments, at least one additional layer of coating on the metal tube can be deposited by repeating the methods described herein.

In some embodiments, at least one additional layer of coating can be deposited by repeating the operations of the methods described herein. In some embodiments of the present methods, after depositing the coating and while heating the metal surface, a vacuum can be re-applied thereto. In some embodiments of the present methods, while heating the metal surface, at least one additional layer of the coating can be deposited by releasing the vacuum and backfilling with a second purge gas.

In some embodiments, the first purge gas and the second purge gas can be the same. That is, in some embodiments, the coating can contain multiple layers in which all the layers are the same. In other embodiments, the first purge gas and the second purge gas can be different. That is, in some embodiments, the coating can contain multiple layers in which at least some of the layers are different. By repeating the operations of the present methods, a coating having any number of layers can be deposited, for example, 1 to about 100 layers, or 1 to about 20 layers, or 1 to about 10 layers, or 1 to about 5 layers, or 1 layer, or 2 layers, or 3 layers, or 4 layers, or 5 layers, or 6 layers, or 7 layers, or 8 layers, or 9 layers, or 10 layers. In embodiments in which multiple layers are present, the various layers of the coating can impart different properties to the metal surface. For example, a first layer can enhance the metal surface's electromagnetic absorptive properties, and a second layer of a different substance can reduce its thermal emission properties.

Various types of coatings can be formed on metal surfaces according to the embodiments described herein. In some embodiments, the coatings can include, for example, at least one of an oxide coating, a nitride coating, a carbide coating, or a fluoride coating. The choice of the type of coating formed on the metal surface can be a matter of the intended use thereof, which will be evident to one having ordinary skill in the art. For example, an oxide coating can be formed by reacting the metal surface with an oxygen-containing purge gas under heating conditions. A nitride coating can be formed by reacting the metal surface with a nitrogen-containing purge gas, particularly molecular nitrogen, under heating conditions. A carbide coating can be formed by reacting the metal surface with a carbon-containing purge gas, including but not limited to organic compounds, under heating conditions. A fluoride coating can be formed by reacting the metal surface with a fluorine-containing purge gas, particularly hydrogen fluoride, under heating conditions. Other types of coatings can be easily envisioned by one having ordinary skill in the art.

Various purge gases and combinations thereof can be used in the embodiments described herein. It is to be recognized that the purge gases suitable for use in the present embodiments can be either substances that are gases at room temperature and pressure or liquids or solids that have a high vapor pressure and are readily volatilized to form a vapor phase. Illustrative purge gases that can be suitable for use in the present embodiments include, for example, air, water vapor, oxygen, carbon dioxide, carbon monoxide, nitrogen, fluorine, chlorine, bromine, iodine, hydrogen fluoride, hydrogen chloride, hydrogen bromide, hydrogen iodide, boron trifluoride, boron trichloride, boron tribromide, silicon tetrafluoride, sulfur hexafluoride, sulfur tetrafluoride, phosphorus trifluoride, phosphorus pentafluoride, nitrogen trifluoride, nitrous oxide, nitric oxide, nitrogen dioxide, dinitrogen tetroxide, diimide, hydrogen, gaseous organic compounds, and any combination thereof

In some embodiments, the purge gas can further include a diluent gas that is not reactive with the metal surface. In some embodiments, the diluent gas can be a noble gas such as, for example, helium, argon, neon, krypton or xenon. When present, the diluent gas can be included in an amount ranging between about 0.1% and about 99.9% of the gas mixture, including all subranges thereof. In some embodiments, the diluent gas can be present in an amount ranging between about 1% and about 90% of the gas mixture, or between about 5% and about 50% of the gas mixture, or between about 10% and about 70% of the gas mixture. Without being bound by theory or mechanism, it is believed that by adjusting the quantity of purge gas that is present to react with the metal surface, by increasing or decreasing the amount of diluent gas, the thickness of the coating on the metal surface can be controlled.

The thickness of the coating on the metal surface can vary over a considerable range. In some embodiments, each layer of the coating on the metal surface can range in thickness between about 1 nm and about 1 μm, including all subranges in between. In some embodiments, a thickness of each layer of the coating can range between about 1 nm and about 250 nm, or between about 1 nm and about 100 nm, or between about 5 nm and about 50 nm, or between about 5 nm and about 100 nm, or between about 10 nm and about 50 nm. That is, the coating can be nanostructured in at least some embodiments.

Suitable temperatures for practicing the present embodiments can likewise vary over a wide range. As one of ordinary skill in the art will recognize, the ultimate temperature range over which the present embodiments can be practiced will depend mainly upon the melting point of the chosen metal surface. Except for low melting point metals (e.g., metals having melting points of less than about 800° C., such as aluminum), suitable temperatures for practicing the present embodiments can vary over a temperature range of about 200° C. to about 1000° C., including all subranges in between. In some embodiments, a suitable temperature can range between about 400° C. and about 800° C. In other embodiments, a suitable temperature can range between about 300° C. and about 600° C. In still other embodiments, a suitable temperature can range between about 400° C. and about 600° C. In some embodiments, a suitable temperature can be at last about 400° C. In other embodiments, a suitable temperature can be at least about 500° C., or at least about 600° C., or at least about 700° C., or at least about 800° C., or at least about 900° C., or at least about 1000° C. It is to be further recognized that factors other than the melting point of the metal surface can also dictate the chosen temperature at which the present embodiments are practiced. For example, certain purge gases may become flammable, explosive, or otherwise unstable if the temperature is excessively high. Thus, the temperature at which a particular coating is prepared according to the present embodiments will be a matter of routine experimental design that is within the capabilities of one having ordinary skill in the art.

In some embodiments, the purge gas can be subjected to a pre-heating operation before being backfilled into a vacuum about a metal surface. Possible reasons one would desire to pre-heat the purge gas can include, but are not limited to, to address cooling of the purge gas due to adiabatic expansion that occurs upon backfilling a vacuum space. Cooling of the purge gas can potentially impact its reaction with a heated metal surface.

Although the invention has been described with reference to the disclosed embodiments, one having ordinary skill in the art will readily appreciate that these embodiments are only illustrative of the invention. It should be understood that various modifications can be made without departing from the spirit of the invention. The particular embodiments disclosed above are illustrative only, as the present invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular illustrative embodiments disclosed above may be altered, combined, or modified and all such variations are considered within the scope and spirit of the present invention. While compositions and methods are described in terms of “comprising,” “containing,” or “including” various components or steps, the compositions and methods can also “consist essentially of” or “consist of” the various components and operations. All numbers and ranges disclosed above can vary by some amount. Whenever a numerical range with a lower limit and an upper limit is disclosed, any number and any subrange falling within the broader range is specifically disclosed. Also, the terms in the claims have their plain, ordinary meaning unless otherwise explicitly and clearly defined by the patentee. If there is any conflict in the usages of a word or term in this specification and one or more patent or other documents that may be incorporated herein by reference, the definitions that are consistent with this specification should be adopted. 

1. A method for depositing a coating on a metal surface, the method comprising: heating a metal surface to a temperature not greater than its melting point; while heating the metal surface, applying a vacuum thereto; and while heating the metal surface, releasing the vacuum and backfilling with a first purge gas; wherein the first purge gas is reactive with the heated metal surface so as to deposit at least one layer of a coating thereon.
 2. The method of claim 1, further comprising: after depositing the coating and while heating the metal surface, re-applying a vacuum thereto.
 3. The method of claim 2, further comprising: while heating the metal surface, depositing at least one additional layer of the coating by releasing the vacuum and backfilling with a second purge gas.
 4. The method of claim 3, wherein the first purge gas and the second purge gas are the same.
 5. The method of claim 3, wherein the first purge gas and the second purge gas are different.
 6. The method of claim 1, wherein the temperature is at least about 400° C.
 7. The method of claim 1, wherein the first purge gas is selected from the group consisting of air, water vapor, oxygen, carbon dioxide, carbon monoxide, nitrogen, fluorine, chlorine, bromine, iodine, hydrogen fluoride, hydrogen chloride, hydrogen bromide, hydrogen iodide, boron trifluoride, boron trichloride, boron tribromide, silicon tetrafluoride, sulfur hexafluoride, sulfur tetrafluoride, phosphorus trifluoride, phosphorus pentafluoride, nitrogen trifluoride, nitrous oxide, nitric oxide, nitrogen dioxide, dinitrogen tetroxide, diimide, hydrogen, gaseous organic compounds, and any combination thereof.
 8. The method of claim 1, wherein the coating comprises at least one of an oxide coating, a nitride coating, a carbide coating or a fluoride coating.
 9. The method of claim 1, wherein the first purge gas further comprises a diluent gas that is not reactive with the metal surface.
 10. A method for depositing a coating on a solar receiver, the method comprising: applying a vacuum to an annulus having an outer surface defined by a material that is at least partially transparent to solar radiation and an inner surface that is defined by a metal tube; heating the metal tube, while applying the vacuum thereto, to a temperature not greater than its melting point; and while heating the metal tube, releasing the vacuum and backfilling with a first purge gas; wherein the first purge gas is reactive with the heated metal tube so as to deposit at least one layer of a coating thereon.
 11. The method of claim 10, further comprising: after depositing the coating and while heating the metal tube, re-applying a vacuum to the annulus.
 12. The method of claim 11, further comprising: sealing the annulus so as to maintain the vacuum therein.
 13. The method of claim 11, further comprising: while heating the metal tube, depositing at least one additional layer of the coating by releasing the vacuum and backfilling with a second purge gas.
 14. The method of claim 13, wherein the first purge gas and the second purge gas are the same.
 15. The method of claim 13, wherein the first purge gas and the second purge gas are different.
 16. The method of claim 10, wherein the material that is at least partially transparent to solar radiation comprises a glass.
 17. The method of claim 10, wherein the temperature is at least about 400° C.
 18. The method of claim 10, wherein the first purge gas is selected from the group consisting of air, water vapor, oxygen, carbon dioxide, carbon monoxide, nitrogen, fluorine, chlorine, bromine, iodine, hydrogen fluoride, hydrogen chloride, hydrogen bromide, hydrogen iodide, boron trifluoride, boron trichloride, boron tribromide, silicon tetrafluoride, sulfur hexafluoride, sulfur tetrafluoride, phosphorus trifluoride, phosphorus pentafluoride, nitrogen trifluoride, nitrous oxide, nitric oxide, nitrogen dioxide, dinitrogen tetroxide, diimide, hydrogen, gaseous organic compounds, and any combination thereof.
 19. The method of claim 10, wherein the coating comprises at least one of an oxide coating, a nitride coating, a carbide coating or a fluoride coating.
 20. The method of claim 10, wherein the first purge gas further comprises a diluent gas that is not reactive with the metal tube.
 21. The method of claim 10, wherein the metal tube comprises a metal selected from the group consisting of carbon steel, stainless steel, and any combination thereof.
 22. A solar receiver prepared by the process of claim
 10. 23. The solar receiver of claim 22, further comprising: a heat transfer fluid located within the interior space of the metal tube.
 24. The solar receiver of claim 22, wherein the coating comprises at least one of an oxide coating, a nitride coating, a carbide coating or a fluoride coating.
 25. The solar receiver of claim 22, wherein the coating comprises a nanostructured coating. 